Adoptive immunotherapy, which capitalizes on generating a potent immune response using genetically re- engineered T cells, represents a novel and powerful therapeutic approach to treat cancer and other diseases. Specifically, patient's T cells are modified to express a chimeric antigen receptor (CAR), which consists of a tumor antigen recognition domain fused to an activation sequence that triggers T cell cytotoxic and proliferative functions when tumor antigen is engaged. The tumor antigen recognition domain of the CAR is commonly a single chain variable fragment (scFv) excised from a monoclonal antibody (mAb) previously selected to recognize a tumor antigen epitope. Herein, we propose to replace the scFv domain with either a fragment antigen-binding domain (Fab) or a mAb that are engineered to bind a cyclic peptide called meditope. Incorporation of this meditope-binding site within the Fab/mAb uniquely marks the modified T cells and permits a highly specific, high affinity interaction with a meditope, an optimized, serum stable peptide that can be conjugated to small molecules including DOTA, biologics including scFvs, chemokines, etc., or nanoparticles. As such, the incorporation of the meditope interface offers the unique and timely opportunity to rapidly and independently add new functionality to CAR T cell therapy. The immediate goal of this application is to demonstrate that a meditope-enabled Fab or mAb can effectively replace the scFv within the CAR and can be used to image meditope-enabled CAR T cells in vivo. Successful demonstration of this novel interaction will set the stage to track meditope-enabled CAR T cells in patients, to correlate T cell distributions with clinical outcomes, and to add new functionality to improve this emerging and broadly applicable therapy.